The invention is directed to adhesive or coating compositions for use with glass and/or metal materials. In particular, the invention is directed to compositions in which multifunctional thiols are used as adhesion promoters and/or primers to enhance the adhesion of photo or electron beam curable polymers, coatings, adhesives or sealants to gold and other precious metals, and alloys thereof.
Adhesives are useful in optical communications for quickly joining elements. In such optical communication applications it is frequently desirable to join together two materials (substrates) having either similar or different coefficients of thermal expansion. When similar materials are joined together, the adhesive is selected to have a coefficient of thermal expansion similar to that of both materials. However, when materials having different coefficients of thermal expansion are used to make a device or element, the selection of an adhesive becomes more difficult. For example, when a device or element is made by joining an optical grating mounted on ultra low expansion (xe2x80x9cULExe2x80x9d) glass to an optical prism made from a glass that has coefficient of thermal expansion different from that of the grating. This combination of grating and prism is frequently called a xe2x80x9cgrismxe2x80x9d. In such an exemplary device it is imperative that the line spacing of the optical grating not change with temperature. For this to be accomplished the adhesive must allow for expansion and contraction of the prism with temperature and must not appreciably transfer excessive force to the grating. In addition, since the adhesive is in the optical light path, it must be transparent at the operating wavelength of the device, for example, at operating wavelengths of 1550 or 1300 nm. Consequently, the adhesive should also be minimally absorbing and minimally scattering (optically clear) at the operating wavelengths. The adhesive should also match as nearly as possible the refractive index of the materials used to make the device. For instance, in the above grism the adhesive should match as closely as possible the refractive index of the prism in order to avoid excessive refraction of the light signal entering or exiting the prism. In the case of the exemplary prism with a refractive index in the range of 1.51 to 1.54 and operating at 1550 nm, the ideal adhesive should have a refractive index in this range.
It is also desirable that the manufacturing of optical devices be as inexpensive as possible. In the early stages of the telecommunications industry devices made of two or more elements were typically made by manually aligning and fusing the elements. Since that time advances have been made through the use of photocurable adhesives. While manual alignment may still occur, the bonding process using adhesives is much simpler than the old fusion process which involves heating the elements to be joined to a temperature at which the material of which they are made is sufficiently soft and flowable so as to join together to form a bond. By using photocurable adhesives the need for such heating and the working of hot materials can be avoided. For example, bringing the grating and prism together with the adhesive in between, optically aligning them, and then curing the adhesive in place by the use of actinic radiation to form the exemplary grism. For example, the curing can be accomplished by shining UV and/or visible light through the prism. The photocure is fast, allowing for perfect alignment, does not involve the use of solvents, does not require the need to heat the adhesive to effect the cure, and does not the require the use of a heating step to joining the grating to the prismxe2x80x94a step which might adversely effect the delicate line structure of the optical grating.
In some applications, optical devices are made from optical elements which themselves are made of different materials. For example, one element may be made of glass and the other made of metal. In such applications precious metals are frequently used because they are resistant to corrosion, rusting and other deleterious effects which may occur during use. When a device is made from one element made of a precious metal and another made of glass, the adhesive must bond to the surface of both the glass and the metal. Further, in order to provide the durability necessary for actual use, the adhesive must maintain its bond to both through accelerated aging cycles commonly used in the industry. For example, accelerated aging under conditions 85xc2x0 C. and 85% relative humidity (xe2x80x9cRHxe2x80x9d) for a minimum of 500 hours, and ideally up to 2000 hours. In addition, the adhesive must maintain its integrity through multiple temperature cycles, typically cycling between xe2x88x9240xc2x0 C. to +85xc2x0 C.
Formulating adhesives or coatings that have a high degree of adhesion to metals, and especially precious metals, is very difficult. The adhesive requirements enumerated precious metal surfaces to other surfaces such as glass. Further, low Tg materials generally do not hold up well under conditions of 85xc2x0 C. and 85% RH.
U.S. Pat. No. 4,497,890 to Herlbert claims a process for improving adhesion of polymeric photoresists to the precious metal gold. Various adhesion promoters were applied as primers to a gold metalized semiconductor substrate. The primers were claimed to enhance the adhesion of a polymeric photoresist to the gold surface during the development process. These adhesion promoters are not sufficient to enable the maintenance of adhesion to long term exposure (i.e.  greater than 500 hours) of 85xc2x0 C. and 85% RH conditions. This is most likely because they were only designed to hold up to the photolithographic development process. The photolithography development process involves an exposure to acidic or basic, aqueous or solvent-based developing solutions. The exposure to the developing solutions is usually quite brief, ranging from a few seconds to several minutes, and is normally done at ambient temperature (approximately 18 to 25xc2x0 C.) or slightly above ambient temperatures. Exposure to 85xc2x0 C. and 85% RH for  greater than 500 hours is a much more aggressive condition.
Japanese Patent JP 10182779 to K. Murakami and T. Isonaka, assigned to Dainippon Ink and Chemicals Inc., discloses UV curable polyurethane-poly(meth)acrylate compositions as adhesives for optical disks with improved adhesion to gold or silicon nitride. These compositions however, contain urethane acrylates with polyester backbones that are susceptible to hydrolysis in the 85xc2x0 C./85% RH accelerated aging tests, and hence are not suitable for optical communication use.
C. G. Khan Malek, and S. S. Das, J. Vac. Science Technology B (1998) 16(6); Pages 3543-2546, describe the enhancement of adhesion of a polymethylmethacrylate (PMMA) layer to gold by either using a novalac resist layer or an amino silane primer. This process involves several heating steps, under nitrogen, up to 180xc2x0 C. and for times up to one hour. Heating at these temperatures is detrimental to many optical communications devices in which adhesives and/or coatings may be used.
Japanese patents JP 59-204676 and JP 59-189178, assigned to Mitsubishi Rayon Co., describe photosensitive adhesives comprising brominated epoxy resin with high refractive index useful for optical devices. However, while these brominated materials bond well to glass, they do not have a low Tg value nor do they have the requisite thermal stability need for telecommunications applications.
M. Manning, Adv. Sci. Technology (1999), 17: pages 681-688 and pages 689-696; British Patent GB 2 289 472; and Japanese Patent JP 0303 1309 describe photocurable adhesives of various compositions for use with optical components. However, such materials, when cured, have high Tg values and a re unsuitable for optical communications devices where low Tg, materials are required.
The art describes thiols and other sulfur compounds reacting with and bonding to gold metal. However, most of these references involve the formation of what are called self-assembled monolayers (SAMS) of the thiol compounds on the gold surface. Normally, the SAMs are deposited onto the gold surface by applying a dilute solution of the thiol in a solvent. The thiol reacts with the gold surface to form an Auxe2x80x94S bond. The solvent is then evaporated leaving a chemically bonded monolayer of the organic compound on the surface of the gold. In some cases, this organic monolayer can enhance the physical adhesion of other organic coating, ink, adhesive, or sealant materials to the gold. The monolayer does not chemically bond to such substances. Consequently, when there is exposure to extremely harsh environments, this physical adhesion is not adequate because there is no chemical bonding between the SAMs and the coating, ink adhesive, or sealant.
Research has also been done using multifunctional thiol materials as ingredients to form SAMs. In these applications, one of the thiol functional groups is chemically attached to the gold surface leaving the other thiol functional group available for further chemical reaction. This approach has been used to attach conductive polymers (J. M. Tour et al., J. Am. Chem. Soc. (1995)117, 9529-9534), metal clusters (J. I. Henderson et al., Inorg. Chim. Acta (1996) 115-124, and R. G. Osifchin, Polymer. Mat. Sci. Eng. (1995), 73, 208-209), and metal ions (W. Deng et al., Jpn. J. Appl. Phys. (2000) 39, L751-754) to gold metal. However, even though organic compounds with mercapto groups have shown the ability to bond to precious metals such as gold, palladium, and platinum, the use of thiols, especially in acrylate resin based adhesives has been difficult. M. Atsuta et al, J. Prosthetic Dent. (1992), 67(3), pages 296-300, state that thiols cause a chain transfer reaction during the propagation of vinyl or acrylic free radicals and considerably effect the degree of polymerization. Atsuta et al further state that if an acrylic mercaptan such as N-(4-mercaptophenyl) methacrylamide is used in an acrylic solvent, gelation of the solution ours during storage. The authors propose that this phenomenon is caused by the reaction of the mercapto group and the acrylic resin compound in solution. Such gelation is undesirable in adhesives and coatings used in optical communications devices because they can cause imprecise alignment, excessive diffraction and excessive absorbance of light.
W. Huang, et al, Langmuir (2001), 17(5), 1731, describe attempts to perform surface initiated thermal free radical polymerization o n gold metal. Azo type free radical initiators were attached to alkane thiol monolayers on gold. This idea was to graft polymer layers onto the gold surface. However, efficient grafting was not possible because desorbed thiols served as efficient chain transfer agents that inhibited the radical polymerization. In addition, the reactive radicals attacked the Auxe2x80x94S bonds that linked the initiator monolayer to the surface.
H. Rieley, et al, J. Chem. Soc. Faraday Trans. (1996) 92, 3629-3634, investigated the chemical behavior of several thiols and disulfides as SAMs ton gold. One of the compounds considered was octane-1,8-dithiol. They found the SAM layers to be susceptible to photo-oxidation and thus recommend that their usage should be restricted to applications where the SAM layer is kept away from light and is used under anaerobic conditions.
C. E. Evans et al., Macromolecules Symposium (1999), 142 (Adv. Polymer Materials), pages 23-31, describe a means of overcoming what they call the inadequate ruggedness of n-alkane, xcfx89-terminated thiol and disulfide based SAMs on gold. By incorporating conjugated di acetylene groups within the monolayer structure and exposing these monolayers to UV radiation, a polymer forms within the single molecular layer through reaction of the acetylene bonds. The photopolymerization of the self-assembled monolayer film results in a conjugated polymer that is very robust and able to withstand temperature, solvent, and electrochemical extremes not previously possible with other monolayer systems. However, this approach requires the extra steps of depositing a thiol-acetylene based material as a primer layer, and then exposing the layer to UV radiation before the UV or electron beam curable material is deposited. The technique also requires a three step chemical synthesis of the conjugated diacetylenic disulfide starting materials. Finally, these materials are often colored due to their conjugated backbone. This coloration is an undesirable characteristic in many optical communications applications and precludes materials from use in optical communications applications.
Omura, et al, U.S. Pat. Nos. 5,064,495 and 5,085,726 assigned to Kuraray Co. Ltd. Corp., and M. Kimura et al, U.S. Pat. Nos. 5,795,497 and 6,133,338 assigned to Tokuyama Corp., disclose mercapto and/or sulfide functional compounds that enhance adhesion to gold and other precious metal alloys. The materials are used either as substrate primers or as ingredients in free radically polymerized adhesive formulations. The formulations are curable at room temperature or they can be thermally or photocured. However, all of the compounds require the presence of at least one olefinic double bond in the mercapto or sulfide functional compound. These materials are also extremely expensive and are thus unsuitable for most optical communications applications.
M. Glodde et al, Internat. J. Adhesion and Adhesives (1998), 18 (5), pages 359-364 and Adhaes-Kleben Dichten (1999), 43(9), pages 36-39, describe using bis-(xcfx89-aminoalkyl) disulfides as adhesion promoters for gold. Such materials were designed for amine cured epoxy resin systems and are not suitable for photo or electron beam curable systems.
Japanese patent JP10176018 by Isonaka et al, assigned to Dainippon Ink and Chemicals, Inc., discloses UV curable adhesive compositions for joining optical disks having thin layers of gold, silicon nitride, or silicon carbide films on their surface. These compositions comprise (meth) acrylic oligomers based on polysulfide backbones. These materials also do not have adequate thermal, oxidative, and hydrolytic stability for the uses contemplated in this application.
Mercapto-esters have been recognized as effective coupling agents for bonding metals using acrylate and epoxy adhesives. R. G. Schmidt et al., J Adhesion (1989), 27, pages 135-142, claimed that the use of these compounds improved the bonding strength and durability of adhesive compositions. D. B. Yang et al., Surface Interface Anal. (1996), 24, pages 803-809, have evaluated such compounds on the surface of gold metal. No adhesion data was given in this study, but the testing described below shows that the mercaptoester functional groups do not hold up well to the 1000-2000 hour exposure to 85xc2x0 C. and 85% RH conditions required for telecommunications devices.
Consequently, despite all the research and development which has been done in the area, there continues a need for adhesive material and compositions that can be used in telecommunications to join optical elements. This invention is invention seeks to fulfill such need.
In one aspect, the invention is directed to the use of multifunctional thiol compounds as adhesion promoters and/or primers to enhance the adhesion of photo or electron beam curable polymers, coatings, adhesives, or sealants to gold, other precious metals, and their alloys.
In another aspect, the invention is directed to the use of actinic radiation and electron beam (xe2x80x9cEBxe2x80x9d) curable compositions for optically clear, low Tg (xe2x89xa630xc2x0 C.), high refractive index ( greater than 1.50 at 1541 nm), thermally, oxidatively, and hydrolytically stable adhesives and/or coatings for glass and/or metal.
In a further aspect, the invention is directed to the use of oligomers or prepolymers formed from the reaction of one or a plurality of multifunctional maleimide compounds with one or a plurality of multifunctional thiols to form materials for use in adhesives or compositions suitable for adhesive use in optical communications.
In an addition aspect, the invention is directed to the use of multifunctional thiols of general formula Rxe2x80x94(SH)n, wherein R is any organofunctional group, excluding polyesters, mercaptoesters, polysulfides and carbon-carbon double bond (xe2x80x94Cxe2x95x90Cxe2x80x94) containing materials, and n is equal to or greater than 2 (nxe2x89xa72). R includes linear, branched and cyclic hydrocarbons, alkylene oxides, alkylene sulfides, alkylene carbonates and alkylene carbamates.
In a further aspect, the invention is directed to the use of adhesives containing maleimide compounds having the general structure 
where X is a linear, branched or cyclic hydrocarbon, alkylene oxide, alkylene sulfide, alkylene carbonate or alkylene carbamate, and m is equal to or greater than 2.
In an additional aspect, the invention is directed photocurable compositions containing, among other substances, a photoinitiator, a dithiol compound, and a polymerizable monomer or oligomer.
The invention is further directed to the addition of a multifunctional thiol component of general formula R(SH)n, where R is any organofunctional group excluding polyesters, polysulfides, mercaptoesters and carbonxe2x80x94carbon double bond containing groups, to curable chemical compositions which are either commercially available or known in the art and which can be used as an adhesive or coating for joining two optical elements or as a coating for an optical element. Such curable compositions must contain a component that is capable of reacting with the thiol functionality (xe2x80x94SH), and such compositions, when cured, is optically transparent at optical communications wavelengths.
The invention is also directed to an optical device an adhesive composition in accordance with the invention located between a first and a second optical element, said adhesive composition being used to join such elements. The composition, when cured, is optically clear, has a low Tg (xe2x89xa630xc2x0 C.), has high refractive index (for example,  greater than 1.50 at approximately 1541 nm), and exhibits thermal, oxidative, and hydrolytic stability making it suitable for use in optical communications. The composition of the invention is an adhesive or coating for glass and/or metal that can be used to join such glass and/or metal.
This invention is further directed to simplified procedures for making multifunction thiols suitable for use in adhesive compositions, and obviates the need for complex chemical synthesis and/or expensive raw materials.
Additional features and advantages of the invention will be set forth in the detailed description and the claims that follow, and will be readily apparent to those skilled in the art from the description or will be recognized by practicing the invention as described and claimed herein.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.